ELECTROKINETIC REMEDIATION OF METAL-CONTAMINATED SOILS

Abstract

The issue of soil pollution has been recognized as a significant concern that poses a threat to human health and well-being. Remediating surface and sub-surface areas that have been contaminated, particularly with heavy metals, is a serious challenge due to the persistent and non-biodegradable nature of metals. The conventional treatment methods for soil cleanup have limitations pertaining to the labor-intensive nature, substantial costs, and time-consuming requirements. Therefore, it is imperative to come up with an innovative technique to overcome these limitations. Electrokinetic Remediation (EKR) is a developing technique to remediate metal contaminated soils. The treatment method involves the application of direct current across the soil, which leads to the migration of contaminants through different transport mechanisms like electromigration, electro-osmosis, and electro-phoresis. EKR has gained much attention in the past decade due to the ability to remove contaminants in situ from a wide range of matrices irrespective of heterogeneity. Although, many researchers have proved its applicability to remove metals in laboratory experiments, and some field tests, there is still a lack of understanding of complicated electrochemical reactions and soil properties upon varying the operating conditions of EKR. Therefore, the present study aims to examine the feasibility of EKR to remediate metal-contaminated soils, particularly, Hexavalent Chromium, Lead, and Cadmium, and to investigate the effect of three main operating variables, applied voltage, electrolyte composition, and electrode material. In addition, the effect of different electrode configurations was also investigated on mixed metal-contaminated soils. The experiments were performed on a laboratory-scale EKR setup. The findings revealed that the application of high voltage (2.5 V/cm) against the lower voltage (1.5 V/cm, as stated in earlier literature) had no significant effect on soil health. Periodic voltage application (Day-on, Night-off) yield better removal and save energy which is an essential factor to take into account for practical applications of EKR. A significant removal of metals was achieved with EKR in a shorter period of 10 h in the order Cr (77%) > Pb (65%) > Cd (30.2%) when a combination of electrolyte amendment with EDTA (0.1 M) with high voltage gradient was employed. The rate of removal only slightly increased when the concentration of EDTA increased from 0.1 M to 0.2 M, iv but the extra input of chemicals added to the cost of the treatment. Thus, the optimum conditions were found to be the combination of high voltage application and 0.1 M EDTA amendment. The application of surfactant as an electrolyte did not improve the efficiency of soil EKR toward the removal of heavy metals. The formation of sharp pH zones in the soil was not found, instead, the pH remained neutral to slightly basic in all experiments. Upon the characterization of soil after EKR treatment for its physical and chemical properties, it was found that the treated soil resulted in improved plastic limit and liquid limit suggesting improved stability of the soil and reduced concentrations of sulphates, further enhancing the soil quality. EKR proved suitable for the treatment of soils contaminated with single metal as well as a multi-metal matrix with almost equal efficiency. The results obtained on comparison of different electrode configurations indicated that the square configuration was able to minimize the inactive electrical zones and yield maximum removal, followed by the trigonal configuration. Considering the efficiency and cost-benefit analysis, it is observed that EKR is suitable for treating the metal-contaminated soil, along with the removal of other organic contaminants, thereby bringing the environmental toxicity down in the soil. The optimization of regulating parameters can not only improve the efficiency of treatment but can also bring the cost of treatment down to a significant level.